1. Field of the Invention
The present invention relates to an apparatus and method for measuring mechanical strength of neck portion of a seed crystal used for growing a silicon single crystal ingot according to the Czochralski method (CZ method), and a method for producing a silicon single crystal.
2. Description of the Related Art:
In the conventional methods of producing a silicon single crystal according to the CZ method, a silicon single crystal is used as a seed crystal, which is brought into contact with silicon melt and then slowly pulled while being rotated to grow a silicon single crystal ingot. In such methods, after the seed crystal is brought into contact with a silicon melt, the so-called necking is performed to form a neck portion having a smaller diameter of around 3 mm. Subsequently, the diameter of the crystal is increased to a predetermined size, and then a dislocation-free silicon single crystal is pulled upwardly.
In this case, the formation of the neck portion enables a single crystal to grow while eliminating dislocation propagated to the straight body following the seed crystal from slip dislocation generated in the seed crystal in high density due to thermal shock which occurs when the seed crystal is brought into contact with silicon melt. However, the crystal weight has recently been getting heavier with use of a larger diameter of single crystals and with the purpose of improving productivity, and therefore mechanical strength of the seed crystal and the neck portion are likely to be insufficient. On the other hand, length of the neck portion itself is getting longer because of automatization of the necking operation and use of larger seed crystals for holding heavier crystals. If the single crystal ingot falls due to breakage of recessed portion of the seed crystal for pin fixation thereof in a seed holder, the neck portion having a small diameter or the like, it may lead to a serious accident.
Therefore, the tensile fracture strength of silicon single crystals becomes important. However, the values of tensile fracture strength of the neck portion determined by a conventional universal tensile tester used for tensile test of metals or the like significantly fluctuate even if the values are sorted out according to the diameters of test pieces corresponding to the thickness of the neck portion, and thus a true value of tensile fracture strength can hardly be obtained. In addition, even if the universal tensile tester is provided with a heater in order to obtain a value under high temperature environment as in an actual single crystal pulling apparatus, highly reliable data cannot necessarily be obtained.
Moreover, because of the characteristic of the crystal, the neck portion is easily destroyed when shearing force (force acting on the transverse direction) is applied, and thus accurate data cannot be obtained.
In particular, under the current circumstance where a longer neck portion as long as 20 cm is used because of the use of larger seed crystal and automatization of the necking operation, the measurement error due to the shearing force has become larger and a problem in obtaining the true value.
Therefore, emphasis on safety as to probability of accidental breakage of a seed crystal or neck portion, which is expected to increase with the growth of larger diameter and larger weight of silicon single crystal, may lead to an excessively high safety factor, and it has reduced the productivity and increased the cost.